"Flat" lighting, such as that from dead ahead, as in on-camera flash, beauty dish, or the old "broad" lighting (nothing to do with gender, but lighting that features the wider planes of the face) will emphasize the largeness of a face, and presupposes a camera angle of ~nose height, which doesn't do anything for a double chin. Baaad idea!
Standard, classic "Rembrandt" portrait lighting, that uses a lighting ratio of 2 or 3 to one, (The main light is 2-3 times as bright as the fill, and provides shadows and modeling of the face, which features the classic triangle of light under the eye.)will slim the face. A slightly high camera angle will minimize a double chin.
How you do that:
Main light; camera (your) right, about an arm's length, or slightly more from you. Light adjusted higher than the subject, to produce the triangle of light just below the subject's right eye. ~45 and 45 works: light 45 degrees between you and the subject, and raised about 45 degrees, to somewhat resemble sunlight. (the original studio light)
Fill light, (or reflector) camera left, Doesn't have to be as high as the main, moved back and forth to provide the lighting ratio on the shadow side of the face. That's the beauty of digital...you can check and adjust immediately.
It's really easier to set up than to tell about it.
Now: the person, the whole reason we are there, the subject, or customer,
Whom we MUST please, if we are to be successful!
No "football shoulders"! Never head-on. Always turn the subject at an angle to the camera.
Everybody has a "Good" side! Find it! Use it! No human being is put together symmetrically. Something will always be slightly larger or higher, etc., than the opposite number.
Don't make it really obvious, but identify the high eye..that goes toward the camera. Always! Focus on the eyes, entertain the subject with intelligent patter, establish rapport, and eh Voila! Ya did it!
Enjoy!
